1. Field of the Invention
The present invention relates to a system for a vehicle for detecting a mobile unit. In particular, the present invention relates to an information system in which drivers are informed of the existence of cars in the same or opposite lanes from each other.
2. Description of the Background Art
Various kinds of detection methods for detecting other cars or other mobile units existing in the vicinity of a car being operated have been offered.
For example, in a method described in the Japanese Patent Laid-open No. Hei 2-216600, a car, being operated while monitoring for warning radio waves includes a receiving device therein. The car is able to detect the existence of other mobile units by the reception of warning radio waves which are being constantly transmitted by transmitting devices provided in the other mobile units.
Furthermore, in a method described in Japanese Patent Laid-open No. Hei 7-244,800, even if the warning radio waves are received as described in the above example, judging from the operating condition of the car, some are not necessary, and only necessary ones are displayed.
In the case where there is a car intending to make a turn crossing over the opposite lane and another car proceeding straight ahead, the driver of the car proceeding straight ahead unable to determine whether or not the driver of the car intending to make a turn has sensed the existence of the car proceeding straight ahead. If the driver of the car proceeding straight ahead operates the car assuming that the driver of the car intending to make a turn has sensed the existence of the car proceeding straight ahead, the operation of the cars becomes speculative.
In the examples mentioned above, the transmitting device provided in the mobile unit is constantly transmitting a warning radio wave. When the number of such mobile units is increased, warning radio waves transmitted by them may cause a jammed state of the radio waves. Furthermore, all of the radio wave will be received in the receiving devices, making the selection of necessary signals difficult.
Furthermore, the transmitting devices are constantly transmitting warning radio waves which in many cases results in useless consumption of energy.